1. Field of the Invention
This invention relates to the upgrading of materials containing oxide of iron in combination with oxide of chromium, for example, chromite. By "upgrading" we mean removal of at least some iron thereby to increase the proportion of chromium. Reference to "material" hereafter, unless otherwise specified, is to the aforesaid material and reference to "iron" or "chromium" unless otherwise specified is to the iron or chromium content of the material or of the upgraded or partially upgraded material present as oxide.
2. Brief Description of the Prior Art
Chromite is a material having a spinel structure based on the theoretical formula FeO.Cr.sub.2 O.sub.3 which is varied in nature by the partial replacement of iron 2+ ions by magnesium 2+ ions and by the partial replacement of chromium 3+ ions by aluminium 3+ or iron 3+ ions. Chromite usually also contains a significant proportion of oxides of silicon and may also contain a small proportion of oxides of some or all of calcium, manganese, niobium, vanadium and titanium.
Chromite is the primary source of chromium for industrial or metallurgical use. There are major ore deposits of chromite on the African continent, particularly in the Transvaal region of South Africa, in the Philippines, in New Caledonia, in Turkey and in the USSR. There are also large deposits of chromite sand in South Africa.
The chromium content of natural chromite deposits varies considerably.
There are large deposits of low grade chromite in which the chromium content, calculated as Cr.sub.2 O.sub.3, is below 50% and in which there is a relatively high iron content giving a chromium to iron ratio which may be below 2:1. Such ores, because of their high iron content, may require processing to upgrade them if they are to be usable, for example, for the production of "ferrochrome" for which use a chromium to iron ratio, generally, of at least 3:1 and, certainly, well above 2:1 is required.
High grade chromite may show chromium contents, calculated as Cr.sub.2 O.sub.3, of 50% to 55% by weight and a chromium to iron ratio well over 3:1. These high grade ores are suitable for the production of chromium/iron alloys containing in the region of about 60% to 70% of chromium commonly known collectively as "ferrochrome". Nevertheless, even high grade ores may have to be upgraded for other purposes such as, for example, the production of a high chromium beneficiate, and it is within the present invention to do so.
The physical form of natural chromite deposits also affects the ease with which they may be exploited. For example, the large deposits of chromite sand in South Africa are composed of relatively low grade chromite and are particularly difficult to upgrade because of their fine particle size.
The problem of increasing the chromium to iron ratio of chromite deposits has been the subject of much investigation which investigation is summarised succinctly in the introductory portion of U.S. Pat. No. 3,216,817. That patent concludes that prior process for the selective chlorination of chromium ores to remove only the iron content is difficult due to the affinity of both iron and chromium for chlorine at high temperatures, that while the use of carbon as a reducing agent together with a chlorinating agent such as chlorine offers the possibility of certain practical advantages it had been found to more readily effect complete chlorination of the chromite ore constituents and consequently to be less conductive to use in selective chlorination methods and that the prior art had theretofore been unsuccessful in achieving selective chlorination with chlorine and carbon in a practical manner to produce an upgraded ore beneficiate having a satisfactory chromium to iron content.
An object of U.S. Pat. No. 3,216,817 is to alleviate the problem noted above. According to the process disclosed in that patent chromium ores may be upgraded by selective chlorination in the presence of carbon in a fluidised bed to convert iron oxide in the ore to ferric chloride, volatile ferric chloride being released from the fluidised reaction mass. According to the process disclosed the reaction temperature must be regulated below 920.degree. C. preferably below 900.degree. C. since a higher temperature results in a rapidly increasing loss of chromium from the ore. The use of an excess of chlorine, which may be a 100% excess or even more, is taught. There is continuously discharged from the reactor a stream containing chlorine, nitrogen and carbon oxides. This gives rise to the dual problem of chlorine recovery from admixture with the nitrogen and carbon oxides as well as from iron chloride formed.
The development of the art of fluidised bed selective chlorination of chromium ores is taken a stage farther by U.S. Pat. No. 3,473,916 which proposes operation at a temperature of from 920.degree. C. to 1050.degree. C. but which finds it necessary to use, as a reducing agent, carbon monoxide in the gaseous input to the fluidised bed. The process conditions disclosed in U.S. Pat. No. 3,473,916 would lead to the conversion of the iron content of the ore into ferric chloride.
It would be desirable to conduct a process for upgrading materials containing oxide of iron in combination with oxide of chromium by a selective chlorination in which iron values are converted to ferrous chloride instead of ferric chloride since thereby a very considerable saving in plant capital costs may be possible due to the lower theoretical chlorine requirement of the process. Ferrous chloride vapour, however, is a difficult to handle material which tends to form solid accretions on the inside surfaces of equipment. Nevertheless, U.S. Pat. No. 2,752,301 describes a process for increasing the chromium to iron ratio of chromite by selectively forming and subliming ferrous chloride. The process described in that patent essentially involves, amongst other features, the use of dry hydrogen chloride as the reagent, the careful avoidance of free chlorine, and the absence, or substantial absence, of carbon the maximum quantity of carbon allowed being 1 part to 6 parts by weight of any iron oxide left in the ore residue at the end of the reaction.
The present process provides, by careful control of a combination of features as taught hereafter, a process for the upgrading of materials containing oxide of iron in combination with oxide of chromium, such as, for example chromite, by means including the chlorination of iron in the ore to ferrous chloride.